A conventional air compressor simply has two air outlet ducts on the gas storage unit, and one duct is used for installing a circular-box pressure display meter and the other duct is connected with a hose having an air nozzle (wherein the air nozzle may be connected to an object to be inflated). Since there are only two air outlet ducts and thus the air compressor is incapable of being installed with a protection or safety device, the inventor of the present invention has developed an air compressor as disclosed in U.S. Pat. No. 7,462,018, wherein the air compressor comes with a structure comprising more than two ducts, wherein a safety valve is provided for relieving the pressure when an over-pressure condition of the air compressor occurs, so as to protect the object to be inflated. Since the conventional circular-box pressure meter is fixed to an external end of the duct directly by screwing means or bolts, and such connection method is undesirable, the inventor of the present invention has developed a pressure meter structure that can be assembled or removed quickly as disclosed in U.S. Pat. No. 8,297,944, and such patented invention allows users to install and remove various devices such as pressure meters, hoses, relief valves, etc, so as to provide a convenient and practical assembling process. However, the conventional air compressor still has a safety valve installed at one of the ducts to prevent the over-pressure issue, thus incurring a higher manufacturing cost for providing the protection effect. On the other hand, the conventional air compressor adopts a pointer-type pressure meter which is a circular-box device, and simply provides the functions of measuring the pressure of the compressed air and displaying the current pressure value. The conventional mechanical circular-box pressure meter does not have other new functions. FIGS. 7 to 9 show a conventional air compressor 7, which is equipped with a safety valve 77 and a circular-box pointer-type pressure meter 76. In operation, after the air compressor is started, a motor 70 drives a transmission mechanism 79 to have a piston conduct reciprocating motion in a cylinder 71 so to produce compressed air in an air reservoir 72 which includes a plurality of air outlet ducts 73, 74, 75, wherein the duct 73 is used for installing an inflating hose (not shown in the figure), the duct 74 is coupled to the circular-box pointer-type pressure meter 76, and the duct 75 is installed with a safety valve 77. Although the functional accessories including the hoses, pressure meter 76 and safety valve 77 can be assembled to the air reservoir 72 of the air compressor 7 directly and conveniently, yet when the compressed air produced in the air compressor 7 is greater than a predetermined safety tire-pressure of a car tire, the safety valve 77 is started to relieve the pressure. If the safety valve 77 fails to work properly, the car tire will be over-inflated or even has a blow-out, and furthermore, the manufacturing cost cannot be lowered.